Method and apparatus for measuring a change in the thickness of polishing pads used in chemical-mechanical planarization of semiconductor wafers

ABSTRACT

The inventive apparatus is a wafer planarizer that has a movable platen, a polishing pad positioned on the platen, a wafer carrier positioned opposite the polishing pad, and a measuring device that is engagable with the polishing pad. The polishing pad decreases as the wafer is conditioned in a straight line correlation with the resulting polishing rate of a wafer. The wafer is attachable to the wafer carrier, and it may be moved with the carrier to be selectively engaged with the polishing pad to planarize the wafer at a wafer polishing rate under a set of wafer polishing operating parameters. After the wafer is planarized, the pad is conditioned by removing a layer of material from the planarizing surface of the pad under a set of conditioning parameters. The measuring device measures the decrease in the thickness of the polishing pad after the pad is conditioned, and it correlates the actual decrease in pad thickness with an expected change in pad thickness. When the same pad conditioning parameters are used from one pad conditioning cycle to the next, a difference between the actual decrease in pad thickness and the expected change in pad thickness indicates a change in operating parameters.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application byScott G. Meikle and Lucky F. Marty entitled "Method for SelectivelyReconditioning a Polishing Pad Used in Chemical-Mechanical Planarizationof Semiconductor Wafers," filed Sep. 29, 1995, now pending.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.08/560,734, filed Nov. 20, 1995, now U.S. Pat. No. 5,609,718; which wasa continuation-in-part of U.S. patent application Ser. No. 08/535,991,filed Sep. 29, 1995, now U.S. Pat. No. 5,655,951.

TECHNICAL FIELD

The present invention relates to an apparatus for measuring a change inthe thickness of polishing pads used in making semiconductor deviceswith chemical-mechanical planarization processes, and a method toindicate an undesirable change in operating parameters.

BACKGROUND OF THE INVENTION

Chemical-mechanical planarization ("CMP") processes are frequently usedto planarize dielectric layers in the production of ultra-high densityintegrated circuits. In a typical CMP process, a wafer is pressedagainst a slurry on a polishing pad under controlled chemical, pressure,velocity, and temperature conditions. Slurry solutions generally containsmall, abrasive particles of silica or alumina that mechanically removethe surface of the wafer, and chemicals that react with the materials ofthe dielectric layers to enhance the removal of the molecules on thesurface of the wafer. The polishing pad is generally a planar pad madefrom a relatively soft, porous material such as blown polyurethane.

CMP processes must accurately planarize the dielectric layer to adesired end-point. Several hundred microelectronic devices are typicallyfabricated on a single wafer by depositing layers of various materialson the wafer, and manipulating the wafer and the other layers ofmaterial with photolithographic, etching, and doping processes. In orderto manufacture ultra-high density integrated circuits, CMP processesmust provide a uniformly planar surface so that the geometries of thecomponent parts of a die may be accurately positioned across the fullsurface of the wafer. Thus, it is important to accurately planarize thewafers to a desired endpoint across the whole wafer.

In the competitive semiconductor industry, it is also highly desirableto maximize the throughput of the CMP processes to produce accurate,planar surfaces as quickly as possible. The throughput of CMP processesis a function of several factors including the rate at which thethickness of the wafer decreases as it is being planarized (the"polishing rate"), and the ability to accurately estimate theplanarizing time to stop the process at a desired endpoint. A highpolishing rate generally results in a greater throughput because itrequires less time to planarize a wafer. Accurately estimating theplanarization time is also important to maintaining a high throughputbecause planarization of a wafer is stopped once the estimatedplanarization time has elapsed to remove the wafer from the pad andmeasure the actual thickness of the wafer. If the thickness of thedielectric layer is not within an acceptable range, the wafer must bere-planarized until it reaches a desired endpoint. Such re-planarizationof a wafer significantly reduces the throughput of current CMPprocesses, so it is very important to provide an accurate estimate ofthe planarization time.

One problem with current CMP processes is that they do not consistentlyproduce a uniformly planar surface at the desired end-point because thewafer polishing rate may change over a number of wafers. Wafer polishingrates change for a number of reasons, and it is difficult to determinewhich one of the wafer polishing operating parameters must be correctedto bring them back to a desired level. Some of the wafer polishingoperating parameters that affect the polishing rate include: (1) thedownward pressure of the wafer against the slurry and pad; (2) therelative velocity between the wafer and the pad; (3) the chemical andabrasive characteristics of the slurry; (4) the condition of the pad;and (5) the temperature of the slurry and the wafer. The downwardpressure of the wafer and the relative velocity between the wafer andthe pad are relatively easy to measure and indicate to the operator. Thecharacteristics of the slurry, the temperature gradient across thewafer, and the condition of the pad, however, are difficult toascertain. For example, a polishing pad will become less effective afterplanarizing a wafer because materials from the wafer and the slurryadhere to the surface of the pad and reduce the pad's ability to abradethe wafer. Polishing pads are consequently "conditioned" to bring themback to their optimal state for planarizing a wafer by abrading theirsurfaces with a diamond-embedded stone. Accordingly, if the polishingrate changes for a reason other than the pressure and velocity, theoperator must guess whether the change was caused by the condition ofthe pad, the temperature gradient across the wafer, the effectiveness ofthe slurry, or some other reason that is not readily indicated on thecontrol panel of the polisher. Therefore, it would be desirable todevelop an apparatus and a method that indicates whether or not thepolishing pad is the reason for a change in the polishing rate.

In addition to identifying which operating parameters have changed, amore fundamental problem with current CMP processes is that it isdifficult to timely ascertain when an undesirable change in operatingparameters has occurred. A change in the operating parameters istypically indicated by measuring the actual change in thickness of anumber of wafers for a given planarizing time, and noting a significantchange in the polishing rates over the number of wafers. Measuring theactual thickness of a wafer, however, is time-consuming because wafersare delicate and require sophisticated, clean-handling procedures. Thus,it would also be desirable to develop an apparatus and a method forindicating when an undesirable change in the operating parameters hasoccurred without having to measure the actual change in thickness of thewafers.

SUMMARY OF THE INVENTION

The inventive apparatus is a wafer planarizer that has a movable platen,a polishing pad positioned on the platen, a wafer carrier positionedopposite the polishing pad, and a measuring device engagable with thepolishing pad. The wafer is attachable to the wafer carrier, and it maybe moved with the carrier to be selectively engaged with the polishingpad to planarize the wafer. The surface layer of the wafer is planarizedat a wafer polishing rate that is controlled by setting the waferpolishing operating parameters. After the wafer is planarized, the padis conditioned by removing a layer of material from the planarizingsurface of the pad. The thickness of the layer of material removed fromthe pad is controlled by setting a number of pad conditioningparameters, and the change in pad thickness over a conditioning cycle isdirectly related to the polishing rate of the next wafer. The measuringdevice measures the decrease in the thickness of the polishing pad aftera pad conditioning cycle, and it correlates the actual decrease in padthickness with an expected change in pad thickness. Thus, when the samepad conditioning parameters are used from one pad conditioning cycle tothe next, a difference between the actual decrease in pad thickness andthe expected change in pad thickness indicates a change in the waferpolishing operating parameters.

In accordance with a method of the present invention, a change in thewafer polishing operating parameters is identified by removing a layerof material from a planarizing surface on the pad according to a set ofpad conditioning parameters. The pad conditioning parameters are set toremove a layer of material having a thickness equal to a desired changein pad thickness. An actual change in pad thickness is then measuredafter the removing step. The actual change in pad thickness is comparedto the desired change in pad thickness, whereby a difference between theactual change in pad thickness and the desired change in pad thicknessindicates an undesired change of the wafer polishing operatingparameters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a chemical-mechanicalplanarization device in accordance with the invention.

FIG. 2 is a schematic cross-sectional view of a measuring device for achemical-mechanical planarization apparatus in accordance with theinvention.

FIG. 3 is a graph illustrating the straight line correlation betweenwafer polishing rates and changes in pad thickness per conditioningcycle.

FIG. 4 is a graph illustrating the polishing rate of wafers.

FIG. 5 is a flow chart illustrating a method in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an apparatus for accurately measuring asmall change in the thickness of a polishing pad, and method foridentifying a change in wafer polishing operating parameters. A centralaspect of the invention is the discovery of a straight line correlationbetween wafer polishing rates and changes in pad thickness perconditioning cycle. The amount of material removed from the pad during aconditioning cycle indicates the roughness of the pad, and thus it alsoindicates the resulting polishing rate of the pad. When the same waferpolishing operating parameters and the same pad conditioning parametersare used from one wafer to the next, the amount of material removed fromthe pad over a given conditioning cycle should be the same from oneconditioning cycle to the next. The apparatus of the invention measuresthe actual change in the thickness of the pad in-situ after a padconditioning cycle. A change in the wafer polishing operating parametersmay be identified by comparing the actual change in pad thickness withan expected change in pad thickness that is based upon the previouschanges in pad thickness per conditioning cycle. The structure of theapparatus and steps of the method of the invention are described ingreater detail below in FIGS. 1-5, in which like reference numbers referto like parts throughout the various figures.

FIG. 1 illustrates a wafer polisher 10 with a platen 20, a wafer carrier30, a polishing pad 40, a measuring device 70, and a conditioning device90. The platen 20 has an upper surface 22 upon which the polishing pad40 is positioned. A drive assembly 26 either rotates the platen 20 asindicated by arrow A and/or reciprocates the platen 20 back and forth asindicated by arrow B. The motion of the platen 20 is imparted to the pad40 because the polishing pad 40 frictionally engages the upper surface22 of the platen 20.

The wafer carrier 30 has a lower surface 32 to which a wafer 60 may beattached. A resilient pad 34 may be positioned between the wafer 60 andthe lower surface 32 to frictionally attach the wafer 60 to the wafercarrier 30. The wafer carrier 30 may be a weighted, free-floating wafercarrier, or it may have an actuator assembly 36 attached to it forimparting axial and rotational motion as indicated by arrows C and D,respectively.

A measuring device 70 engages at least one area on the polishing pad 40to measure the change in the thickness of the pad 40. The measuringdevice 70 is preferably a laser position sensor or a laserinterferometer with an emitter 82 and a detector 84. The emitter 82shoots a laser beam 83 onto a planarizing surface 42 of the polishingpad 40. The laser beam 83 reflects off of the planarizing surface 42 andtravels back up to the detector 84. Other measuring devices such as acapacitance meter that measures the capacitance of the polishing pad 40or a stylus are also within the scope of the invention. The measuringdevice 70 preferably engages the surface of the pad after the pad isconditioned, and accurately measures the change in thickness of the pad40 to within 0.2 μm. Accordingly, the measuring device 70 providesin-situ measurement of the change in thickness of the polishing pad 40.

The conditioning device 90 is positioned over the pad 40. Theconditioning device 90 has a disk 92 with an abrasive surface forengaging the planarizing surface 42 of the pad 40. In operation, thedisk 92 rotates while it engages the planarizing surface 42, and theconditioning device 90 moves across the polishing pad 40 in a directionindicated by arrow E as the pad 40 rotates on the platen 20. Theconditioning device 90 selectively removes a layer of material from theplanarizing surface 42 of the pad 40 to create a new planarizingsurface. The thickness of the layer of material removed from the pad iscontrolled by setting a number of pad conditioning parameters. The padconditioning parameters generally include the elapsed time of theconditioning cycle, the abrasiveness of the disk 92, the downforce ofthe conditioning device 90, and the material from which the pad 40 ismade. As explained in detail below, when both the wafer polishingoperating parameters and the pad conditioning parameters are constantfrom one conditioning cycle to the next, the thickness of materialremoved from the pad should also be the same for each conditioningcycle. The pad 40 is preferably conditioned with the conditioning device90 after each wafer is planarized.

FIG. 2 further illustrates a laser position sensor with a laser emitter82 and a detector 84. The laser beam 83 initially reflects off theoriginal planarizing surface 42(a) of the pad 40 to an originaldetection point 88(a) on the detector 84. As the thickness of the pad 40decreases because of conditioning, the surface of the pad 40 moves downto a subsequent planarizing surface 42(b). The distance between theoriginal planarizing surface 42(a) and subsequent planarizing surface42(b) is the change in thickness of the pad 40 (ΔT). The laser beam 83reflects off the subsequent planarizing surface 42(b) and travels to asubsequent detection point 88(b) on the detector 84. The detectionpoints 88(a) and 88(b) are separated by a distance "d." By knowing theangle of inclination θ of the laser beam 83 with respect to the surfaceof the pad 40 and the distance "d" between the detection points 88(a)and 88(b), the change in thickness ΔT of the pad 40 may be calculated asfollows:

    ΔT=(d) Sin θ.

Once the difference in the thickness of the pad ΔT is determined, anundesired change in wafer polishing parameters may be identified.

The measured change in pad thickness ΔT caused by conditioning indicatesan undesired change in wafer polishing operating parameters when thesame pad conditioning parameters are used for each conditioning cycle,but the measured change in pad thickness for the latest cycle isdifferent from those of previous cycles. Referring to FIG. 3, the changein pad thickness per conditioning cycle is directly related to the waferpolishing rate in a straight line relationship. For example, a change inpad thickness of ΔT₁ μm/cycle corresponds to a wafer polishing rate ofP₁ Å/s, and a change in pad thickness of ΔT₂ μm/cycle corresponds to awafer polishing rate of P₂ Å/s. Since the wafer polishing rate isaffected by a change in the wafer polishing operating parameters, andthe change in pad thickness per conditioning cycle is related to thewafer polishing rate, a change in the wafer polishing operatingparameters will also affect the change in pad thickness per conditioningcycle. In accordance with the discovery of the invention, unless achange occurs in either the pad conditioning parameters or the waferpolishing operating parameters, the change in pad thickness perconditioning cycle should remain the same over a large number ofconditioning cycles. Thus, when a pad is conditioned using the same padconditioning parameters for each conditioning cycle, a difference in thechange in the pad thickness from one conditioning cycle to anotherindicates that a change in the wafer operating parameters has occurred.

One advantage of the present invention is that provides a timelyindication that the wafer polishing operating parameters have changedwithout measuring the planarized wafers. In accordance with theinvention, only the change in pad thickness per conditioning cycle needsto be measured to indicate that a change has occurred in the waferpolishing operating parameters. Since the wafer planarizer 10 of theinvention measures changes in pad thickness in-situ and at eachconditioning cycle, a change in the wafer polishing operating parametersmay be indicated between each wafer polishing and without touching thewafers themselves.

Another advantage of the present invention is that it may be used toselectively recondition a polishing pad to obtain a desired waferpolishing rate as disclosed in the related application set forth above.Once a difference in the change in pad thickness per conditioning cycleis measured from one cycle to the next, the pad conditioning parametersmay be changed to selectively recondition the pad to obtain a desiredpolishing rate. For example, if a desired wafer polishing rate of P₁ Å/scorresponds to a change in pad thickness per conditioning cycle of 1.0μm/cycle, and if 5.0 μm of material is removed from a pad during aconditioning cycle, then the pad conditioning parameters could be resetduring the next few pad conditioning cycles until only 1.0 μm ofmaterial is removed from the planarizing surface of the pad at eachconditioning cycle. It will be appreciated that the desired waferpolishing rate of P₁ Å/s will be obtained once the change in padthickness per conditioning cycle is 1.0 μm/cycle only if the other waferpolishing operating parameters have in fact remained the same. Thus, bychanging the pad conditioning parameters to remove the desired amount ofmaterial from the pad, the desired wafer polishing rate may be obtainedwhen the other wafer polishing operating parameters have remained thesame.

More specifically, the wafer polisher 10 may be used to identify whetheror not the condition of the pad 40 is the source of a change in thepolishing rate of the wafer. FIG. 4 illustrates an example of a typicalchange in the polishing rate in a CMP process over a number of wafers.Up until the number of wafers polished equals N_(x), the polishing rateof the wafers is constant at P₂ Å/s. After the number of polished wafersexceeds N_(x), the polishing rate begins to drop off precipitously suchthat when the number of polished wafers equals N_(y) the polishing rateis equal to P₁ Å/s. The pad 40 can generally be isolated as the reasonfor a change in the wafer polishing rate if the actual change in padthickness, as measured by the planarizer 10 of the invention,corresponds to the expected change in pad thickness as determined by thepolishing rate and the straight-line relationship shown in FIG. 3. Forexample, once the polishing rate of the wafer 60 changes at N_(y), thechange in pad thickness in conditioning cycle would be estimated to beΔT₁ μm/cycle according to the straight line correlation shown in FIG. 3.If the actual change in pad thickness as measured by the waferplanarizer 10 is equal to ΔT₁ μm/cycle, then the change in the polishingrate was most likely due to the conditioning of the pad. If, however,the actual change in pad thickness is not reasonably close to ΔT₁μm/cycle, then the actual change in pad thickness indicates that the pad40 is still in good condition and is likely not the reason for thechange in the polishing rate. Therefore, the wafer planarizer 10 canmore particularly indicate whether or not the condition of the pad 40 isthe source of a change in the polishing rate of the wafer 60.

FIG. 5 schematically illustrates a method of identifying whether achange in the wafer operating parameters has occurred. The first step102 is to remove a layer of material from a planarizing surface on thepad to condition the pad according to a set of pad conditioningparameters. The pad conditioning parameters are desirably kept constantfrom one conditioning cycle to the next so that the pad conditioningparameters can be eliminated as the source of any difference in theamount of material that is removed from the pad from one conditioningcycle to another. Accordingly, an expected change in pad thickness isgenerally equal to the change in pad thickness of prior conditioningcycles. The second step 104 is to measure an actual change in padthickness since the last conditioning using the wafer planarizer 10described above with respect to FIGS. 1 and 2. The third step 104 is tocompare the actual change in pad thickness with the expected change inpad thickness. A difference between the actual change in pad thicknessand the expected change in pad thickness indicates that a change in thewafer operating parameters has occurred.

While the detailed description above has been expressed in terms of aspecific example, those skilled in the art will appreciate that manyother structures could be used to accomplish the purpose of thedisclosed procedure. Accordingly, it can be appreciated that variousmodifications of the above-described embodiment may be made withoutdeparting from the spirit and scope of the invention. Therefore, thespirit and scope of the present invention are to be limited only by thefollowing claims.

What is claimed is:
 1. A method of identifying a change in polishingoperating parameters in planarization of a semiconductor wafer against apolishing pad in which the polishing pad is conditioned by removing alayer of material from a planarizing surface on the pad according to aset of conditioning parameters, the method comprising the stepsof:measuring an actual change in pad thickness caused by removing thelayer of material from the planarizing surface of the pad; and comparingthe actual change in pad thickness with an expected change in padthickness based upon a polishing rate of previous wafers and apredetermined correlation between wafer polishing rates and changes inthickness per conditioning cycle, whereby a sufficient differencebetween the actual change in pad thickness and the expected change inpad thickness indicates a change in a wafer polishing operatingparameter.
 2. The method of claim 1, further comprising evaluating anactual polishing rate of the wafer with the expected polishing rate ofthe wafer based on previous wafers, and determining that a waferoperating parameter other than the polishing pad condition has changedwhen the actual polishing rate of the wafer is sufficiently differentthan the expected polishing rate and the actual change in pad thicknessis approximately equal to the expected change in pad thickness.
 3. Themethod of claim 1 wherein a wafer planarizer with a measuring device fordetermining a change in pad thickness is provided, and wherein themeasuring step further comprises engaging the measuring device with thepolishing pad after each conditioning cycle.
 4. The method of claim 3wherein the measuring device is an interferometer and the engaging stepcomprises impinging the polishing pad with a laser beam and sensing achange in phase of a reflected portion of the beam with a detector. 5.The method of claim 3 wherein the measuring device is a laser positionsensor and the engaging step comprises impinging the polishing pad witha laser position sensor and sensing a change in position of the laserbeam with a detector.
 6. In planarization of substrates, a method ofidentifying whether a polishing pad is a source of a change in apolishing rate of a substrate, the method comprising the stepsof:determining an actual polishing rate of the substrate; estimating anexpected change in pad thickness after conditioning of the pad accordingto the actual polishing rate of the substrate and a predeterminedcorrelation between substrate polishing rates and changes in padthickness per conditioning cycle for a set of conditioning parameters;removing a layer of material from a planarizing surface on the pad tocondition the pad according to the set of conditioning parameters;measuring an actual change in pad thickness caused by removing the layerof material from the planarizing surface of the pad; and comparing theactual substrate polishing rate with an expected substrate polishingrate, and comparing the actual change in pad thickness with the expectedchange in pad thickness, wherein the polishing pad is likely a source ofa discrepancy between the actual polishing rate and the expectedpolishing rate when the actual change in pad thickness is approximatelyequal to the expected change in pad thickness.
 7. A wafer planarizer,comprising:a moveable platen; a polishing pad positioned on the platen,the polishing pad having a thickness that decreases when the pad isconditioned; a wafer carrier positioned opposite the polishing pad, awafer being attachable to the wafer carrier whereby the wafer isengageable with the polishing pad to planarize the wafer at a waferpolishing rate under a set of operating parameters; and a measuringdevice engagable with the polishing pad, the measuring deviceperiodically measures the actual decrease in the thickness of thepolishing pad caused by conditioning the pad and correlates the actualdecrease in pad thickness with an expected change in pad thickness,whereby a difference between the actual decrease in pad thickness andthe expected change in pad thickness indicates a change in the operatingparameters.
 8. The planarizer of claim 1 wherein the measuring device isan interferometer.
 9. The planarizer of claim 1 wherein the measuringdevice is a capacitance meter.
 10. The planarizer of claim 1 wherein themeasuring device is a laser position sensor.
 11. The planarizer of claim1 wherein the measuring device engages a specific area on the polishingpad each time the actual decrease in pad thickness is measured.
 12. Theplanarizer of claim 1 wherein the measuring device is able to measure achange in thickness of the pad to approximately within 0.2 μm.
 13. Theplanarizer of claim 1, further comprising a conditioning devicepositioned over the pad for periodically reconditioning the pad.
 14. Asubstrate planarizer, comprising:a moveable platen; a polishing padpositioned on the platen, the polishing pad having a thickness thatdecreases when the pad is conditioned; a substrate carrier positionedopposite the polishing pad, a substrate being attachable to thesubstrate carrier whereby the substrate is engageable with the polishingpad to planarize the substrate at a substrate polishing rate under a setof operating parameters; and an indicator having a measuring device thatmeasures the actual decrease in the thickness of the polishing padcaused by conditioning the pad and compares the actual change in padthickness with an expected change in pad thickness to indicate a changein an operating parameter when the actual change in pad thickness issufficiently different than the expected change in pad thickness. 15.The planarizer of claim 14 wherein the measuring device is ainterferometer.
 16. The planarizer of claim 14 wherein the measuringdevice is a capacitance meter.
 17. The planarizer of claim 14 whereinthe measuring device is a laser position sensor.
 18. The planarizer ofclaim 14 wherein the measuring device engages a specific area on thepolishing pad each time the actual decrease in pad thickness ismeasured.
 19. The planarizer of claim 14 wherein the measuring device isable to measure a change in thickness of the pad to approximately within0.2 μm.
 20. The planarizer of claim 14, further comprising aconditioning device positioned over the pad for periodicallyreconditioning the pad.
 21. A substrate planarizer, comprising:amoveable platen; a polishing pad positioned on the platen, the polishingpad having a thickness that decreases when the pad is conditioned; asubstrate carrier positioned opposite the polishing pad, a substratebeing attachable to the substrate carrier whereby the substrate isengageable with the polishing pad to planarize the substrate at asubstrate polishing rate under a set of operating parameters; and a padcondition identifier having a measuring device that measures the actualdecrease in the thickness of the polishing pad caused by conditioningthe pad, correlates an actual polishing rate of a substrate with anexpected decrease in pad thickness after conditioning the pad based on apredetermined relationship between polishing rates and changes in padthickness during conditioning, and compares the actual decrease in padthickness with the expected decrease in pad thickness, wherein thepolishing pad is identified as a cause of a difference between theactual polishing rate and an expected polishing rate based on previoussubstrates when the actual decrease in pad thickness is approximatelythe same as the expected decrease in pad thickness for the actualpolishing rate of the substrate.
 22. The planarizer of claim 21 whereinthe measuring device is an interferometer.
 23. The planarizer of claim21 wherein the measuring device is a capacitance meter.
 24. Theplanarizer of claim 21 wherein measuring device is a laser positionsensor.
 25. The planarizer of claim 21 wherein the measuring deviceengages a specific area on the polishing pad each time the actualdecrease in pad thickness is measured.
 26. The planarizer of claim 21wherein the measuring device is able to measure a change in padthickness of the pad to approximately within 0.2 μm.
 27. The planarizerof claim 21, further comprising a conditioning device positioned overthe pad for periodically reconditioning the pad.